1. Field of the Invention
This invention relates to a fluorophosphate optical glass having optical constants, i.e. refractive index (nd) of 1.54 to 1.60 and Abbe number (.nu.d) of 68 to 75 and having abnormal partial dispersion represented by a relative partial dispersion of at least 0.537.
2. Description of the Prior Art
An optical glass having abnormal partial dispersion, capable of effecting correction of the secondary spectrum in dispersing a lens, is very important for designing the same and is of high utility value.
As to fluorophosphate glasses having abnormal partial dispersion and a refractive index (nd) of at most 1.50, a number of reports have been made and it is known that the fluorophosphate glass disclosed in Japanese Patent Laid-Open Publication No. 144141/1988 has been produced on a commercial scale and used for various optical instruments.
On the other hand, optical designers have eagerly had requirements for an optical glass having a higher refractive index, lower dispersion and abnormal partial dispersion. The optical glasses having optical constants satisfying such requirements are known as disclosed in Japanese Patent Publication Nos. 28169/1978, 4145/1985 and 14500/1987.
The known optical glasses contain large amounts of Ba(PO.sub.3).sub.2 and BaO as essential components as in a glass composition, for example, comprising 2.0 to 13.0% of MgF.sub.2, 0 to 20.0% of SrF.sub.2 1.0 to 33.0% of BaF.sub.2, 0 to 8.0% of AlF.sub.3, 0 to 8.0% of YF.sub.3, 13.0 to 39.0% of Al(PO.sub.3).sub.3, 0 to 24.0% of Mg(PO.sub.3).sub.2, 1.0 to 20.0% of Ca(PO.sub.3).sub.2, 5.0 to 19.0% of Ba(PO.sub.3).sub.2, 10.0 to 36.0% of BaO, 1.5 to 12.0% of Y.sub.2 O.sub.3 and/or Yb.sub.2 O.sub.3, 0 to 6.0% of ZnO, 0 to 29.0% of PbO and 0 to 22.0% of Nb.sub.2 O.sub.5, % being by weight, described in Japanese Patent Publication No. 4145/1985. However, the large contents of Ba(PO.sub.3).sub.2 and BaO result in not only increase of the dispersion but also lowering of the abnormal partial dispersion. These known optical glasses having a relatively high liquidus temperature are so unstable that crystals tend to be precipitated and comparable properties to those of the optical glass according to the present invention cannot be realized in a stable manner.